1. Field of the Invention
The present invention generally relates to an optical transmitter for use in optical communication systems and, more particularly, to a high-power optical transmitter including a rare earth-doped fiber.
2. Description of the Related Art
An optical amplifier directly amplifying an optical signal as it is, without converting the optical signal into an electrical signal, is substantially bit rate-free and facilitates construction of a large-capacity system. Because of such features, intensive studies are now being made in various research institutions on the optical amplifier as one of the key devices in the optical communication system for future. As one of the types of such optical amplifier, there is an optical amplifier using an optical fiber, chiefly the core of which is doped with a rare earth element (such as Er, Nd, and Yb), hereinafter called "rare earth-doped fiber".
The optical fiber amplifier has such excellent characteristics that it provides high gain, its gain is not dependent on polarization, it produces low noise, and it incurs little connection loss at its connection with an optical fiber as a transmission line. Because of this, a high-power optical transmitter can be realized by using the optical fiber amplifier as the booster amplifier.
FIG. 1 is a block diagram of a conventional high-output optical transmitter provided with an optical fiber amplifier. Signal light from a signal light source 2 such as a laser diode is passed through an optical isolator 4 in the forward direction and led into a multiplexer 8. In the multiplexer 8, the signal light and pumping light from a pumping light source 6 such as a laser diode are multiplexed, and the signal light and the pumping light are introduced into a rare earth-doped fiber 10. The signal light amplified in the rare earth-doped fiber 10 is passed through an optical isolator 12 in the forward direction and transmitted over an optical transmission line, not shown.
According to the arrangement of the above optical transmitter, the signal light is amplified in the rare earth-doped fiber 10 on the principle of stimulated emission at the wavelength of the signal light by the pumping light and, therefore, a high-output optical transmitter is realized. However, there has been a problem with such conventional optical transmitter incorporating an optical fiber amplifier that its structure is complicated, though it can output signal light of high power.